This invention relates to capacitive pressure transducers having a stationary electrode and a movable diaphragm and, more particularly, to a capacitive pressure transducer having a tensioned diaphragm.
Capacitive pressure sensors typically include a stationary electrode having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor and a deformable conductive member, such as a metal foil diaphragm, which forms the other plate of the capacitor. Generally, the diaphragm is edge-supported, having a central portion that is movable and positioned substantially parallel to and opposite the fixed plate. Since the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance of the sensor is inversely proportional to the gap, d, between the central portion of the diaphragm and the conductive surface of the stationary electrode. In order to provide a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed or pneumatically separated from the region on the opposite side.
In practice, the geometry and physical characteristics of the diaphragm are selected so that the pressure differentials across the diaphragm in a particular range of interest cause predetermined displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates (i.e. the electrode and the diaphragm), and thus in capacitance variations produced by the transducer. For relatively high sensitivity, such transducers require large changes of capacitance in response to relatively small pressure changes.
One type of pressure transducer includes a tensioned diaphragm that is peripherally supported by the peripheral rim of a concave body member of the transducer. This type of pressure transducer is difficult and expensive to manufacture because the manufacturing process requires that the diaphragm be maintained in tension while it is secured to the body member. FIGS. 1A and 1B show one method of fastening a diaphragm under tension to a single concave body member. One way of effecting this method includes fastening the diaphragm to a stretching frame and pressing the frame over the peripheral rim of the concave body member in order to create the appropriate tension. The stretched diaphragm is then fastened to the peripheral portion of the concave body. However, the diaphragm under tension cannot be fastened by a continuous weld since during the welding process, the tension in the central portion releases as the welded portion melts. One method of fastening the stretched diaphragm to the single concave body member is to apply a sequence of overlapping spot welds so that upon completion, overlapping spot welds fasten the diaphragm to the concave body member and create a seal. The peripheral portion of the diaphragm that extends beyond the concave body member would be removed after welding. Alternatively, the diaphragm could be held in place with a plurality of tack welds distributed along the peripheral rim of the concave body member and a continuous weld could be applied to peripheral rim at a location outside of the tack welds to seal the diaphragm to the concave body member. However, these methods are not capable of producing a pressure transducer that includes a pair of concave body members because spot welding does not effectively secure the two concave body members together with the diaphragm in between and the additional heat required produce a continuous weld between the two concave body members (which are typically 100 to 200 times the thickness of the diaphragm) would cause the portion of the diaphragm tack welded to one of the concave body members to melt releasing the tension in the diaphragm.
This method has many disadvantages. The diaphragm has to be oversized and thus some of the material is wasted. The process requires the precursor step of fastening the oversized diaphragm to the stretching frame. The overlapping spot welding process used to secure the diaphragm to the concave body member is labor intensive and time consuming. Finally, further processing is required to remove the excess diaphragm after welding.
Accordingly, it is an object of the present invention to provide an improved method of producing a pressure transducer.
Another object of the present invention is to provide an improved pressure transducer.
Yet another object of the present invention is to provide an improved pressure transducer that is relatively inexpensive and easy to manufacture.
Still another object of the present invention is to provide an improved pressure transducer having a tensioned diaphragm.
A fuirther object of the present invention is to provide an improved pressure transducer having a tensioned diaphragm in which the tension in the diaphragm can be adjusted.
A still further object of the present invention is to provide an improved pressure transducer having a suitable pressure port.